The principle of quantum transport in Nb-In0.75Ga0.25As-Nb Josephson junctions is the phase coherent Andreev reflections at the interfaces of the Nb and 2DEG in In0.75Ga0.25As, where the microscopic phase of the quasiparticles wavefunction in In0.75Ga0.25As and the macroscopic phase of the superconducting order parameter, are combined [1]. The conductance in this system is influenced by several kinds of interference effects. For instance, the conductance oscillates in a step- or sinusoidal- manner when the hybrid junctions are placed in a magnetic field which gives rise to Aharonov-Bohm type effect observation even in such a simply connected system.Here, we report the first experimental observation of periodic conductance oscillation in ballistic Nb-In0.75Ga0.25As 2DEG-Nb junctions in the low magnetic field. We demonstrate that the dI/dV (B) has a maximum close to zero field which drops in a step-like manner when the field is increased.When the field is reversely swept, a hysteresis is observed which is accompanied by oscillation of dI/dV in a sinusoidal manner with a periodicity of ~15 mT at and below T= 320 mK.[1] K. Delfanazari, et al., Adv. Mater. 29, 1701836 (2017).

Van der Waals (vdW) materials exhibit a wide range of electronic phenomena and phases such as charge density wave, metal-insulator transition, magnetism and superconductivity. Heterostructures based on these materials enable studies of the proximity effects of correlated phases in ultra clean atomically smooth interfaces. In this talk, we present our spatially resolved measurement of induced superconducting gap and coherence length, in single and a few layer vdW materials proximitized by superconducting niobium diselenide in zero and finite magnetic fields.

Cd3As2 is a 3D topological Dirac semimetal with non-trivial Fermi-arc surface states. It has been suggested that topological superconductivity can be achieved in the Fermi arcs by utilizing the superconducting proximity effect. Here we report a series of first observations of supercurrent states in Al-Cd3As2-Al Josephson junctions. First, the junction reaches a zero-resistance state at temperatures below 0.8K. Second, a non-monotonic B-field dependence is observed for the critical current Ic, where Ic is at first enhanced by increasing magnetic fields. In one sample, Ic increases from 4.6μA at B=0 to 4.8μA at B=5mT and further increasing the B field to 40mT eventually quenches Ic. Third, Ic exhibits a weak π-periodic Josephson effect at a temperature below 0.2K. Finally, Shapiro steps are also observed in a.c. Josephson effect measurements. Taken together, our results suggest that an unconventional electronic state is realized in the proximity-induced superconducting Cd3As2.

At metal-superconductor interfaces, Andreev processes occur where an electron tunneling into the superconductor carries with it a second electron, effectively reflecting a hole with opposite momentum back into the metal. This is due to the superconducting gap, which, at low energies, only allows the formation of cooper pairs inside the superconductor, representing an accessible way to measure Cooper-pair tunneling phenomena. An important requirement for strong Andreev processes is a clean interface with a high transmission probability. Graphene is a promising candidate for achieving an extremely clean interface to superconductors, however recent results show achieving a transparent interface is non-trivial. In this work, we use controlled assembly in inert atmosphere to create high-quality interfaces between graphene and superconductors. With dual graphite gated graphene, low disorder broadening around charge neutrality point (CNP) could be achieved, which gives opportunities to understand Andreev processes which happen near CNP. In addition, large upper critical fields of 2D superconductors allow us to reach different quantum hall states in graphene while preserving superconductivity. In this work, we describe the resultant Andreev processes observed at such interface.

Superconductivity and quantum Hall effect (QHE) are distinct states of matter arising in complementary physical conditions. Recent theoretical developments suggest that coupling at the interface of a QHE edge state with a superconductor (SC) can provide a fertile ground for realizing exotic topological excitations such as non-abelian Majorana, parafermion or Fibbonacci particles. As a step toward that goal, we demonstrate in this letter Andreev reflection (AR) at the junction of a QHE state in a single layer graphene (SLG) and a two dimensional (2D) NbSe2 superconductor. This system allows us to study Andreev effect up to magnetic fields as high as B = 10T when graphene consists of well resolved Landau levels (LL). Our principal result is the observation of an anomalous finitetemperature peak located precisely at the Dirac point, which provides a definitive evidence for inter-Landau level Andreev reflection. We also find other characteristic signatures of AR, such as enhanced conductance inside the superconducting gap and oscillations in the conductance as a function of the magnetic field or the chemical potential. Our observations are well supported by detailed numerical simulations, which offer additional insight into the role of the edge states in coupling SC and QH state.

We show that it is inevitable that time reversal symmetry breaks in a superconducting tunneling junction formed by two superconductors with different pairing symmetries. While the leading conventional Jospheson coupling vanishes in such a junction, the forth order in tunneling always generate chiral superconductivity (SC) orders with broken time reversal symmetry. The topology of all possible TRB SC orders are analyzed and only the d + id′ order has a nonzero Chern number. The self-consistent mean field calculation is deployed on two layered t − J model to justify our viewpoints. A robust d ± id′ order appears in our setup.

I will discuss the renormalization of the charging energy in a topological superconductor islands connected to a ground by a weak link. I will show analytical results on the almost transparent junction and numerical results compounding the analytical conclusions. I will discuss the consequences of the results to the design of topological qubits and scalable quantum computer architectures.

The nonlinear Josephson inductance is an essential element in many quantum device applications including superconducting qubits. Semiconductor proximity junctions could provide additional functionality to these devices, such as gate-tunable critical currents or exotic Majorana fermions via strong spin-orbit coupling. We investigate Joesphson inductance of Indium Arsenide proximity junctions embedded in a microwave resonator based on aluminum coplanar waveguide structures. Our results on the nonlinear inductance and dissipation as a function of the current and the top-gate voltage are discussed.

We investigate the crucial role of interface on superconducting proximity and its inverse effect, using epitaxially grown vertical/lateral superconductor/normal metal (S/N) heterostructures. The two different S/N heterostructures have utterly different interface nature: one is transparent and the other opaque. Because of the electrically transparent interface of the vertical heterostructure, electrons fully coherence in the vertical S/N heterostructure. Local tunneling spectroscopy and global superfluid density measurement show the uniform superconducting gap (SCG) throughout whole vertical S/N heterostructure with no discontinuity. In contrast, lateral S/N heterostruture reveals the spatial dependent SCG with an abrupt discontinuity due to the diffuse interface. Moreover, we found that for the vertical heterostructure in Cooper limit, electron-phonon coupling strength weakens as the thickness ratio of Pb to Ag is lower than 2.

We have developed superconducting Ti transition edge sensors with Au protection layers on the top and bottom. The bottom Au layer (deposited on a thin Ti glue layer) isolates the Ti from any substrate effects; the top Au layer protects the Ti from oxidation during processing and subsequent use of the sensors. We control the transition temperature of the sensors by varying the relative thicknesses of the Ti and Au layers. Films derived from these studies have been successfully deployed to the South Pole Telescope as part of the SPT-3G experiment. Here, we present new work exploring the underlying physics associated with these films. We show that our measured films are well-described by a N-S-N model derived from the Usadel equations (where we ignore the bottom Ti glue layer). From this model we infer that the transition temperature is roughly six times more sensitive to the thickness of the bottom Au layer than to that of the top Au layer. We present preliminary results investigating this asymmetry.

Nanoscale superconductor-normal metal single-electron turnstile is one of the promising devices for metrological applications, such as a quantum current source. In this device, electrons are transferred one-by-one across a mesoscopic metallic island. This control of transport at the single electron level is achieved due to the Coulomb interaction of electrons on the island and the sharp energy gap of the superconducting electrodes. We investigate the main error mechanisms that place the ultimate limit on the accuracy of this device. While multi-electron tunnelling processes limit the accuracy, and are therefore detrimental to the operation of the single-electron devices in metrological application, they perform the key function in information processing applications. An example is a three-terminal Cooper-pair splitting device with a superconducting electrode tunnel coupled to two normal metal electrodes. We employ the Nambu-Gor’kov and Schwinger-Keldysh formalisms to describe the nonequilibrium transport properties of the device for arbitrary transmissions of the barriers and for a general electromagnetic environment. We derive the analytic expressions for the current and the nonlocal differential conductance, and analyze the limits of clean and dirty superconductivity.

We present data on superconducting tunneling van der Waals heterostructures fabricated by stacking the superconducting transition-metal dichalcogenide (TMD), NbSe$_2$ with insulating BN. Such devices are of interest due to the atomically-flat and clean interfaces achievable in van der Waals heterostructures. In comparison, the interface uniformity of high quality Al/AlOx/Al junctions, which are often used to make superconducting qubits, depend on Al and AlOx grains with sizes between 10-100nm leading to uncertainty in the junction's characteristics. Additionally, the performance of Al/AlOx/Al junctions can degrade over short time scales with exposure to an oxygen environment or as oxygen diffuses outwards from the tunnel barrier. We reveal ways in which encapsulated S-I-S junctions in van der Waals heterostructures can be expected to improve upon these difficulties.